Electromagnetic induction-based direct melting can rapidly melt metallic materials, thereby ensuring high yield with minimized contamination of raw materials. The electromagnetic induction-based direct melting is generally performed according to the following principle.
When an alternating current is applied to an induction coil wound around a crucible to induce magnetic field variation, an induction current is created on the surface of metal to be melted and generates Joule's heat, which melts the metal. Further, the induction current interacts with a magnetic field to generate the Lorentz force in molten metal.
Since the Lorentz force is always directed toward an inner center of the crucible and provides a pinch effect or electromagnetic pressure effect according to Fleming's left hand rule even when the direction of the current in the coil is varied, it is possible to prevent the molten metal from contacting an inner wall of the crucible.
However, the electromagnetic induction melting cannot be applied when melting semiconductors such as silicon. That is, since silicon has a very high melting point of 1,400° C. or more and a very low electric conductivity at 700° C. or less unlike metals, it is difficult to achieve direct electromagnetic induction-based silicon melting.
When melting the semiconductors such as silicon, indirect melting with heat from a graphite crucible is generally used. Although graphite is a non-metallic material, it has very high electric and thermal conductivity, thereby allowing the crucible to be easily heated through electromagnetic induction.
As known in the art, however, since graphite in the crucible shields electromagnetic waves, an electromagnetic force cannot be delivered into the graphite crucible. Thus, currently, melting the semiconductors such as silicon in the graphite crucible is carried out only by indirect melting with heat from the crucible.
When indirect silicon melting is performed in the graphite crucible, silicon melt contacts the surface of the crucible. Then, the silicon melt reacts with graphite, thereby causing carbon contamination on silicon from the inner surface of the crucible. Furthermore, the reaction between the silicon melt and graphite generates a silicon carbide layer on the inner surface of the crucible, which often causes cleavage of the crucible.
To solve such problems, Japanese Non-Examined Patent Publication No. 2005-281085 (published on Oct. 13, 2005) discloses a technique for silicon carbide (SiC) coating or high density treatment on an inner surface of a graphite crucible which will contact silicon. FIG. 1 shows a cross-section of the graphite crucible, an inner surface of which is coated with SiC.
In FIG. 1, a silicon carbide coating 110 is formed on the inner surface of the graphite crucible and suppresses reaction between graphite and silicon melt. As a result, it is possible to prevent contamination of silicon or the crucible. Furthermore, the suppression of the reaction can prevent thickness growth of a composite layer 120, which has silicon carbide dispersed in a graphite matrix of the composite layer 120, into a graphite base 130, thereby preventing cleavage of the graphite crucible.
However, the SiC coating 110 tends to be exfoliated from the inner surface of the crucible while melting silicon in the crucible, thereby reducing lifespan of the crucible and insufficiently preventing contamination of silicon.
A cold copper crucible can be used to prevent contact between the silicon melt and the inner surface of the crucible during silicon melting. However, although this crucible has a merit of preventing contact between the silicon melt and the crucible by electromagnetic induction, this crucible requires an assistant heat source for forming an initial silicon melt and generally undergoes severe heat loss due to cooling water.
To solve the problems of the cold copper crucible, Japanese Non-Examined Patent Publication No. 2001-19594 (published on Jan. 23, 2001) discloses a technique of using plasma as an assistant heat source. However, this technique complicates structure of a silicon melting apparatus and provides low efficiency due to heat loss of 30% or more through cold copper crucible.
To solve the problems of the graphite crucible and the cold copper crucible, Korean Patent Laid-open Publication No. 10-2006-0016659 (published on Feb. 22, 2006) discloses a crucible which combines the structure of the cold copper crucible (cold crucible) and the graphite crucible (hot crucible). The structure of this crucible is shown in FIG. 2.
In FIG. 2, the disclosed crucible includes a hot crucible 250 formed of a graphite material and disposed on top of a cold copper crucible 220. The hot crucible 250 has a circumferentially integral upper end and plural segments 240 are formed from a lower end of the hot crucible 250 to a lower end of the cold crucible 220 by a plurality of vertical slits 230. The hot crucible 250 is insulated by an insulator 260 to improve silicon heating efficiency and to protect an induction coil 210.
In the crucible of this configuration, after forming an initial silicon melt using the hot graphite crucible 250, a raw material of the initial silicon melt is further heated and melted, with electromagnetic pressure longitudinally exerted to the overall silicon melt and maintained above the hydrostatic pressure of the silicon melt, thereby improving heating and melting efficiency.
Since the disclosed crucible is formed by combining the cold crucible and the hot crucible, it is more difficult to fabricate such a combination type crucible than an integral type crucible such as the graphite crucible and the like. Moreover, as shown in FIG. 2, since the upper hot crucible formed of the graphite material serves only as the assistant heat source and silicon melting is performed substantially by the cold crucible, the crucible inevitably undergoes heat loss due to water cooling.
Furthermore, although the conventional cold copper crucible can prevent the silicon melt from contacting the inner wall of the crucible by the electromagnetic induction melting, it still suffers contamination of silicon or the crucible due to contact between the silicon melt and an inner bottom surface of the crucible.